The method of pressurizing a liquid product of an air separator and then evaporating it is frequently also termed "internal compression". Processes of this type have long been known for the production of a constant rate of a pressurized gas (for example DE-C-752439) and offer the advantage of lower apparatus costs in comparison with the gaseous product compression.
"Alternating reservoir processes" having at least two reservoir tanks are likewise known, in which variable rates of an atmospheric gas can be produced under atmospheric pressure and, nevertheless, steady-state operation of the rectification is possible (see, for example, W. Rohde, Linde-Berichte aus Technik und Wissenschaft, 54/1984, pages 18 to 20).
The publications DE-B-1056633, EP-A422974, EP-A-524785 and EP-A-556861 indicate processes which combine internal compression and alternating reservoir storage by buffering in reservoir tanks both the liquid product to be evaporated and the heat-transport medium (air or nitrogen) liquefied in the evaporation. The problem of the varying requirement of heat-transport medium for the evaporation of the liquid product is solved in DE-B-1056633 by expanding, so as to perform work, the respective portion of the heat-transport medium which is not required for the evaporation and discarding it. Later processes moved away from this and, instead, variable amounts of heat-transport medium are compressed (EP-A-422974, EP-A-524785 and EP-A-556861). Whereas in the first case a purified gas is lost unused, in the second case large relative fluctuations of the compressor throughput occur. The two types of plant can only be run in the respective operating mode.
The object therefore underlying the invention is to specify a process and an apparatus which can be operated as flexibly as possible and which avoid, in particular, the above-described disadvantages.